1. Field of the Invention
The present invention relates to a dielectric thin film (e.g., for a capacitor in a semiconductor apparatus) and a fabrication method thereof, and in particular to an improved dielectric thin film and a fabrication method thereof capable of preventing leakage current in operation by providing a dielectric thin film.
2. Description of the Conventional Art
As well known to those skilled in the art, a unit device of a Dynamic Random Access Memory (DRAM) consists of one transistor and one information storing capacitor. The storing capacity of the information storing capacitor has a storing capacity of at least 30 fF/cell so as to prevent a soft error operation which is caused by .alpha.-particle.
The more the integration level of the DRAM apparatus is increased, the more the problem for increasing storing capacity in a limited cell space occurs. In order to overcome the problem, the following methods are introduced in the industry based on an expression of (1). EQU C=.di-elect cons..sub.o .di-elect cons..sub.r A/d (1)
where C denotes a storing capacity of a battery, .di-elect cons..sub.o denotes a dielectric constant in vacuum, .di-elect cons..sub.r denotes a dielectric constant of a dielectric thin film, A denotes a surface of a battery, and d denotes a thickness of a dielectric thin film.
The first method is to make the thickness of a dielectric thin film thinner, the second method is to increase the effective surface of a capacitor, and the third method is to use a certain material having a greater dielectric constant.
Among theses methods, the first method has a disadvantage in that it is difficult to adapt to a large scale memory device since the reliability of the thin film is decreased due to the Fowler-Nordheim current in case that the thickness of a dielectric layer is below 100 .ANG..
The second method has disadvantages in that manufacturing process has complexity, and manufacturing cost is increased. That is, although various kinds of capacitor cells, having a three dimensional construction, such as a stack type, a trench type, a fin type, and a cylindrical type are used in 4 MB DRAM and 16 MB DRAM, the above-mentioned capacitor cells can not be effectively used in 64 MB and 256 MB DRAMs.
In addition, the trench type capacitor has a disadvantage in that a leakage current between trenches occurs due to a scaling down operation, and since the cell of the stack type, the fin type, and the cylindrical type have many curved portions and step coverage on the surfaces thereof in order to obtain more storing capacity, it is difficult to perform a photolithography process in the follow-up process, and mechanical strength of the thin film and the cylindrical capacitor is very weak.
The third method is directed to using a high dielectric constant material such as Y.sub.2 O.sub.3, Ta.sub.2 O.sub.5, TiO.sub.2, etc, and recently, a ferroelectric material such as a PZT(PbZr.sub.x Ti.sub.1-x O.sub.3) or a BST(Ba.sub.x Sr.sub.1-x TiO.sub.3 are used. Generally, the ferroelectric material has a self-polarizing characteristic and a dielectric constant from a few hundreds to 1,000. Using this ferroelectric material, it is possible to fabricate a desired thin film having an equivalent oxide thickness of below 10 .ANG. even forming a ferroelectric film having a thickness of a few hundreds.
In particular, the BST among the ferroelectric materials is proper as a high dielectric constant material for a DRAM capacitor since it has a high dielectric constant even in a high frequency as compared to a PZT, and fatigue and aging phenomenon are removed since it is converted to a paraelectric material in accordance with a ratio of Ba/Sr.
Conventionally, a sputtering method, a metal-organic chemical vapor deposition method (MOCVD), a spin coating method, or an aerosol method is used for integrating a high dielectric constant material in a semiconductor apparatus. Sputtering generally involves four steps: ions are generated and directed at a target; the ions sputter target atoms; the ejected (sputtered) atoms are transported to the substrate; and the sputtered atoms condense and form a thin film.
However, the problems occurring while fabricating a BST thin film for use of the DRAM capacitor is that a dielectric constant is decreased since the thickness of a dielectric thin film becomes thin and a more leakage current occurs at a real operation voltage level. In this case, it is possible to prevent the decrease of the dielectric constant through a fabrication process, however, since the phenomenon that the leakage current is decreased and the breakage magnetic field is reduced are the inherent problems of the material, it is the most important problems in really using the BST thin film in the industry.
Therefore, when fabricating the capacitor of the semiconductor apparatus using the BST thin film, the reliability of the semiconductor apparatus is decreased due to a characteristic deterioration of the capacitor.
To improve the above-mentioned problems, a method for improving a dielectric characteristic of the BST thin film which is so bulky and used in a separate device of a ceramic capacitor is disclosed in the industry, which is characterized to add other materials to the BST thin film.
For example, the U.S. Pat. No. 4,058,404 discloses a method for increasing the dielectric constant and for preventing a dielectric variation in accordance with applied voltage. The above method is characterized to insulating using SrTiO.sub.3 of about 18-70%, BaTiO.sub.3 of about 4-7%, Bi.sub.2 O.sub.3 of 2.5-17.5%, and either NiO, Al.sub.2 O.sub.3, or Cu.sub.2 O.
In addition, the U.S. Pat. No. 4,888,246 discloses another method for improving the reliability of the high dielectric constant material and for increasing breakage electric field of a dielectric thin film having a Nitrogen-contained BST by adding Nitrogen into a sputtering gas when depositing a thin film in a sputtering method and the Perovskite structure of AMO.sub.3.